1. Field of the Invention
The invention relates to an anti-pinch sensor, particularly for detecting an obstacle in the path of an actuating element of a motor vehicle.
2. Description of the Background Art
Conventional anti-pinch sensors utilize, for example, the capacitive measuring principle to detect an obstacle. In this case, an electric field is created between a measuring electrode and a suitable counter electrode. If a dielectric enters this electric field, the capacitance of the capacitor formed by the measuring electrode and the counter electrode changes. In this way, theoretically, an obstacle in the path of an actuating element of a motor vehicle can be detected, provided its relative dielectric constant ∈r differs from the relative dielectric constant of air. The obstacle in the path of an actuating element is detected without physical contact with the anti-pinch sensor. If a change in capacitance is detected, countermeasures, such as, for example, stopping or reversing of the drive, can be initiated in a timely fashion, before actual pinching of the obstacle occurs.
In the case of actuating elements of a motor vehicle, this may refer, for example, to an electrically actuated window, an electrically actuated sliding door, or an electrically actuated trunk lid. An anti-pinch sensor, based on the capacitive measuring principle, may also be used to detect an obstacle in the case of an electrically actuated seat.
Non-contact anti-pinch sensors, based on the capacitive measuring principle, are known, for example, from European Pat. Applications Nos. EP 1 455 044 A2, which corresponds to U.S. Publication No. 20040172879, and EP 1 154 110 A2, which corresponds to U.S. Pat. No. 6,337,549. These prior-art anti-pinch sensors generate an external electric field by means of a measuring electrode and a suitable counter electrode, so that a dielectric entering this external electric field may be detected as a change in capacitance between the measuring electrode and counter electrode. To be able to assure a high reliability in the detection of pinching, in addition in the two prior-art anti-pinch sensors the distance between the measuring electrode and counter electrode is designed as flexible, as a result of which physical contact between an obstacle and the anti-pinch sensor can also be detected as a change in capacitance.
European Pat. Application No. EP 1 371803 A1, which corresponds to U.S. Pat. No. 6,936,986 discloses an anti-pinch sensor based on the capacitive measuring principle. In this case, a sensor electrode, which is connected via a shielded line to an evaluation unit, is used to generate an electric field within the opening range of the actuating element. The electric field is generated here relative to the body of a motor vehicle as the counter electrode.
A disadvantage of the prior-art anti-pinch sensors, based on the capacitive measuring principle, is the risk of misdetection of pinching, when there is dirt or water on the sensor. Dirt or water also leads to an altered capacitance, so that a conclusion on an instance of pinching would be erroneously reached.